December, 1923
1NDUSTRlAL A N D ENGINEERING CHEMISTRY
and composition of the seed wheat have no effect upon the per cent of yellow-berry or the protein content of the resulting crop; furthermore, that proper crop rotation and proper seed-bed preparation have a beneficial effect upon milling and baking value of the wheat; and that apparently some varieties are more resistant t o the common factors which usually result in increased percentage of yellow-berry. It is probably not necessary to state that crop rotation and proper seed-bed preparation bring sufficient yield increase to pay several times the additional cost of production, even though an improvement in milling and baking quality were not secured.
Flour Manufacture By C. H. Bailey UNIVERSITY OF MINNESOTA, ST. PAUL,MINN
HE success of the roller milling process is dependent upon the fact that substantial differences exist in the physical properties of the several wheat kernel structures. Thus, the branny covering of the berry is distinguished by its tough, fibrous character. The germ, or embryo, is rich in fats and oils, and hence tends to flake when crushed between rollers. The endosperm, or floury portion, on the other hand, is friable and tends to fracture when struck. It is impossible, however, to effect a quantitative separation of the various kernel structures in roller milling. Small quantities of the fibrous bran and the oily germ find their way into the flour, and particularly into the low or clear grades. A substantial proportion of endosperm or floury material is likewise to be found in the by-products, bran and middlings. Commercial bran not infrequently contains 15 to 20 per cent of flour, while commercial standard middlings are often found to contain 30 per cent or more of flour. It accordingly follows that, of the 82 to 85 per cent of endosperm (potentially flour) to be found in the average plump wheat kernel, not to exceed 7 2 to 75 per cent of the kerntsl is separated in the form of straight grade flour. I n the modern roller mill the first important process is cleaning and conditioning the grain. Commercial wheat frequently contains a considerable quantity of foreign matter, including weed seeds, sticks, stones, fragments of straw, etc. An effort is made to separate these from the wheat as completely as possible before the latter is ground. Northwestern grown hard spring wheat contains more of this foreign matter or dockage than does any other class of American wheat, and a variety of cleaning appliances are necessary to the separation of the foreign matter in this wheat. Certain of the weed seeds, which are most difficult of separation, are of interest to the chemist because of constituents which they contain. Thus the seed of the corn cockle, Agrostemma githago, contains a saponin which is reported to be somewhat toxic. The seed of the wild vetch, Vicia angustifolia, is very yellow, owing to the carotinoid pigments which it contains, and a cyanogenetic glucoside found in this seed yields hydrocyanic acid and benzaldehyde when treated with warm water. If wild vetch seed is present in wheat in excess of 1 per cent a t the time of milling, the characteristic odor of benzaldehyde will be observed in the dough when the flour is mixed with water. Seeds of the giant ragweed, commonly known as “king heads” in the grain trade, if present in wheat a t the time it is ground, result in flour of less desirable properties. It is accordingly necessary that such seeds be separated before attempting to grind the wheat.
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The spores of bunt or stinking smut are frequently found in commercial grain, particularly from western areas, and relatively small amounts of this material will darken t h e flour appreciably. Several special treatments are resorted to in order to remove these smut spores, including scouring with bolted air-slacked lime, and washing with liberal quantities of water. I n the latter event special machinery must be available for “whizzing” the wheat in a centrifugal drier to remove the excess of water. It is a common practice to add appreciable quantities of water to dry wheat before grinding, in an effort to enhance the toughness of the branny covering. This water may be added in the wet washing process or may be sprayed upon the grain in what is commonly known as the tempering process. Frequently, cold wheat is warmed slightly before the tempering water is added. The moistened grain is held for a few hours in order that the bran may absorb the added water, but the period is so adjusted that no substantial penetration of the water into the endosperm results. The cleaned and tempered wheat is then crushed by passing it between corrugated steel rollers. The individual rollers of the pair revolve a t different rates of speed, the differential being commonly about 2.5 to 1. After the first breaking or crushing of the wheat, the resulting chop is bolted and the coarse material reground between another pair of corrugated break rolls. Five or six breaks are ordinarily provided in hard wheat mills, and after the last break the coarse material is known as bran. In addition to the coarse fractions scalped from the break roll chop, two other classes of materials are separated on bolting this chop. The fine portion which will pass through a bolting silk is known as break flour, and is sprouted directly to the flour bins. Fragments intermediate in size between the coarse fraction and the flour are known as “middlings.” These are further classified by passing them over a sieve in a machine known as a purifier, which effects a separation on the basis of size of particles. In addition, a fan a t the top of the purifier box draws an air current through the sieves, which lifts out light, fibrous particles. The purified middlings are then ground between smooth steel rollers and the ground material is bolted to remove the fine particles of flour, Coarse fractions which failed to pass through the flour sieves are reground twice or three times. The flour produced by each regrinding of this material contains increasing proportions of fiber, ash, and soluble proteins, and becomes progressively darker in color. From this description of the process it is evident that a considerable number of flour streams will be found in a flour mill, one such stream resulting from each break and each middlings reduction. Not uncommonly twenty t o twenty-five or more flour streams are thus available to the miller, and he may combine any or all of them to suit the demands of the trade, While the individual flour streams may be bleached, the more common process when bleaching is resorted to is to treat the finished products with bleaching agents. The most common bleaching agents used in American inills are: (1) Nitrogen peroxide, ordinarily generated in the mill by means of a flaming electric arc. ( 2 ) Chlorine, which is usually purchased in a liquid form in steel drums, often mixed with small percentages of nitrosyl chloride. Chlorine is generated by the electrolytic process in a few mills. Occasionally, a double treatment with chlorine and anhydrous ammonia is resorted to. (3) Nitrogen trichloride produced commercially by passing gaseous chlorine through a solution of ammonium sulfate. (4) Benzoyl peroxide, added in mixture with acid calcium phosphate below the second break roll.
Numerous criteria of flour grades have been proposed by cereal chemists during the past half-century. Among
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 15, No. 12
these the following may be mentioned: ( a ) ash content, The work referred to has convinced the writer that the (b) pentosan content, (c) fat content, ( d ) fiber content, ( e ) physical properties of the gluten are, in many instances a t titrable acidity of water extracts, (f) percentage of soluble least, the determining factor as to whether or not a given proteins, (9) specific conductivity of water extracts, (h) lot of flour will produce a satisfactory loaf of bread. It buffer action of water extracts, (i) catalase activity, ( j ) would appear that the physical properties of the gluten relative proportion of branny particles, and (IC) number of are in a large measure determined at the time the wheat wheat hairs per unit of flour. proteins are laid down in the endosperm of the wheat berry, All the foregoing bear a negative correlation t o the grade with the result that the strong flours are strong, not beof flour-that is, the percentage or index increases with cause they differ in acid or salt content from weak flours, decreasing grade or degree of refinement and commercial but because the gluten is capable of forming a more tenacious value. I n addition to the items mentioned, flour grade is gel than is the gluten from a weak flour. The question as generally correlated with the color or visual appearance of to why glutens differ in tenacity is still unsolved. Neverdry flour, of dough made therefrom, and of the baked bread. theless, the fact remains that such differences do occur. The appearance of the crumb of baked bread is extensively It would appear as if the flour strength problem were closely used in cereal laboratories as an index of flour grade. The allied to the rubber problem. Why does rubber produced color which results on wetting the flour is used in a crude in different localities and by different methods differ so way by millers and milling chemists in what is known as enormously in tensile strength and in value? It appears the Pekar test. This test has been made more or less quan- probable that rubber is a polymer of isoprene. It may be titative by measuring the relative extent of color change that chemically the rubbers are indistinguishable from per unit of time in the freshly wetted material. each other, but this does not preclude a wide difference in physical properties. A solution of the rubber problem may solve the strong and weak flour question, and perhaps the cereal chemist may solve the problems vexing the rubber manufacturer. It is a long cry from bread t o automobile tires; nevertheless, in this problem of tensile strength they as Systems’ meet on common ground. I n an investigation having for its object the question as By Ross Aiken Gortner t o which of the proteins of wheat flour was responsible for the variation in the physical properties of the gluten, it was UNIVERSITY OB MINNESOTA, ST. PAUL, MI“. found3 that the alcohol-soluble protein apparently showed HEAT flour, the only one of the cereal flours which uniform physico-chemical properties in both strong and is adapted to the manufacture of yeast-leavened weak flour; the glutenin, however, differed widely from bread, owes its desirable properties to the nature Aour t o Aour in its physico-chemical properties. It appears, of the proteins of the wheat. These proteins have been therefore, that the question of flour strength is dependent extensively studied and comprise a prolamine gliadin, a primarily on the physical properties of the glutenin. Viscosglutelin glutenin, an albumin leucosin, a globulin, and a ity measurements on flour-in-water suspensions brought to a pH of 3.0 by the addition of lactic acid have been found proteose. The gliadin and glutenin are present in the dough in a to serve admirably as a means of evaluating gluten quality, rather intimate physical or physico-chemical mixture which and it has been found possible by such measurements to is known as the gluten. Whether or not gluten is formed in give for the first time a numerical value to gluten quality. the process of dough manufacture or whether it already ex- Inasmuch as viscosity is such an important property to the ists in the endosperm of the wheat berry is still an open emulsoid colloids, i t appears probable that the colloidal properties of the gluten gel are the determining factors question. The gluten plays its role in bread manufacture in that it in flour strength. It is possible to destroy the desirable absorbs water and forms an elastic gel, which stretches baking qualities of wheat flours by altering the physical under the teniion of the carbon dioxide produced by yeast properties of the gluten gel. For example, doughing u p a fermentation and thus “raises” the loaf. The tenacity flour with 70 per cent alcohol and immediately evaporating with which gluten particle adheres to gluten particle varies the alcohol at room temperature will ruin a wheat flour widely from flour to flour. Those flours which have a high for bread-making. On the other hand, such treatment is ratio of carbon dioxide production to carbon dioxide diffusing without effect upon rye flour. Both wheat and rye flour through the gluten are desirable for bread-making purposes, contain the same prolamine gliadin, but rye flour contains and are known as “strong” flours. Those flours that form no glutenin, and these experiments are regarded as addidoughs which fail to retain a large proportion of the carbon tional evidence that it is the physico-chemical properties of dioxide are known as “weak” flours, and are more suited to the protein glutenin which cause the variation in the baking the manufacture of pastry and crackers than to yeast- qualities of wheat flour. (Variations in baking quality may be caused by factors other than gluten quality. The disleavened bread. Many investigators have concerned themselves with a cussion in this paper is limited to gluten quality and is not study of the factor$ which may be involved in flour strength, intended to include factors influencing yeast activity or varibut it has been only recently that the newer theories and ations due to different amounts of gluten.) Colloid phenomena are likewise concerned in the baking methods of colloid chemistry have been applied. These studies give promise of assisting materially in a solution of the processes and in the baked loaf. During the baking process the proteins are heat-coagulated, causing changes in their problem.2 water-holding capacity. A part of the starch grains are 1 Abstract of paper. The complete paper expanded to cover a someruptured, causing the gelatinized starch to absorb moisture, what broader field will appear a s a chapter in a book on colloid chemistry the net result of such changes being that the consistency of which is to be issued under the editorship of Jerome Alexander. 1 For a review of the literature dealing with flour strength and the the dough is altered to t h e rather firm texture of the baked original papers on the relation of flour strength to colloidal properties, see loaf. When such a loaf is stored at ordinary temperature
Flour and Bread
Colloid
W
J . d g v . Research, 13, 389 (1918);J P h y s . Chem., 26, 101 (1922);27, 481, 567, 674,771,in press (1923).
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J . P h y s . Chem., 27, 674 (1923).
